1. Technical Field
The present invention relates to a non-cyanide displacement gold plating solution and, more particularly, to a non-cyanide displacement gold plating solution which is suitable when gold plating is performed on an electrolytic nickel plated substrate by displacement.
2. Background Art
In recent years, large-scale integration of semiconductor devices has advanced rapidly, and accordingly the substrate of printed wiring board etc. that mounts the semiconductor devices has come to have a high wiring density. For such a wiring board, for example the parts-mounting portions, terminal portions, etc. of a wiring circuit are plated with gold to improve the solderability and wire bonding ability. In this case, nickel-based plating is generally performed as a substrate for gold plating.
As the nickel-based plating for forming the substrate for gold plating, electrolytic nickel plating, electroless nickel-phosphorus plating, electroless nickel-boron plating, and the like are known. For the gold plating performed on the substrate, electroless plating rather than electrolytic plating is often used because a uniform plating thickness can be obtained despite small gaps, intricate shapes, etc.
As an electroless gold plating solution, a solution using gold cyanide complex has typically been known, and also various kinds of non-cyanide gold plating solutions have been proposed from the viewpoint of the influence of recent environmental issues and the like.
In some of the non-cyanide gold plating solutions, sulfite, mercaptosuccinic acid, ethylene diamine, etc. are used as a complexing agent. The non-cyanide gold plating solution is broadly divided into a displacement gold plating solution for displacement of a substrate metal and a so-called electroless gold plating solution for depositing a gold film by using a reducing agent.
With the conventional displacement gold plating solution, there has been a tendency such that although an electroless nickel-phosphorous plated substrate can be subjected to displacement gold plating, an electroless nickel-boron plated substrate cannot satisfactorily be plated with gold. That is, the conventional displacement gold plating solution cannot satisfy the requirements for practical characteristics such as gold plating adhesion properties and solderability.
In the case where the substrate is electrolytic nickel plated, it is said to be very difficult to cause a displacement reaction of nickel by gold using a conventionally proposed cyanide or non-cyanide displacement gold plating solution because of the influence of nickel oxides. Also, even if displacement gold plating can be performed on the electrolytic nickel plated substrate, the adhesion thereof is very poor, and a practical level of solderability cannot be realized.
The present invention has been made in view of the above situation, and accordingly an object thereof is to provide a non-cyanide displacement gold plating solution with which even an electroless nickel-boron plated or electrolytic nickel plated substrate can be subjected to displacement gold plating with good adhesion properties and high solderability.
To achieve the above object, the inventors conducted studies earnestly on sulfite-based displacement gold plating solution, and resultantly the inventors came up with an idea for a displacement gold solution in accordance with the present invention.
The displacement gold plating solution in accordance with the present invention is characterized by containing 0.01 to 1.0 g/L (expressed in terms of gold concentration) of sodium gold sulfite or ethylene diamine complex of sodium gold sulfite, 10 to 100 g/L of sulfite, 5 to 50 g/L of organic carboxylic acid or salt thereof, and 5 to 50 g/L of ethylene diamine tetraacetic acid or salt thereof, and having a pH value in the weak acid range of 4.5 to 6.0.
The outstanding feature of the displacement gold plating solution in accordance with the present invention is that the gold concentration is as low as 0.01 to 1.0 g/L and the pH value is in the weak acid range of 4.5 to 6.0. According to the displacement gold plating solution in accordance with the present invention, not only an electroless nickel-phosphorous plated substrate but also an electroless nickel-boron plated substrate can be subjected to displacement gold plating with good adhesion properties and high solderability. The displacement gold plating solution in accordance with the present invention enables even an electrolytic nickel plated substrate to be subjected to displacement gold plating that has good adhesion properties and satisfies practical requirements for solderability.
For the displacement gold plating solution in accordance with the present invention, the gold compound in the plating solution is contained in a form of sodium gold sulfite or ethylene diamine complex of sodium gold sulfite, and the gold concentration thereof is 0.01 to 1.0 g/L. If the gold concentration is lower than 0.01 g/L, the amount of gold deposited by displacement is very small, so that the plating time must be impractically prolonged. Also, if the gold concentration is higher than 1.0 g/L, there is a tendency toward poor adhesion.
The displacement gold plating solution in accordance with the present invention contains three kinds of complexing agents of sulfite, organic carboxylic acid or salt thereof, and ethylene diamine tetraacetic acid (hereinafter referred to as EDTA) or salt thereof. In the present invention, sulfite is referred to as a basic first complexing agent, and organic carboxylic acid or salt thereof and EDTA or salt thereof are referred to as a second complexing agent.
For the sulfite, which is the first complexing agent, SO32xe2x88x92 therein serves to restrain excessive oxidation of nickel and thereby steadily accelerating the displacement of nickel by gold. The sulfite preferably has a concentration in the range of 10 to 100 g/L. If the concentration is lower than 10 g/L, the effect of restraining oxidation is small. If the concentration is higher than 100 g/L, the elution of nickel is excessively restrained, so that the displacement gold plating inversely has poor adhesion.
The organic carboxylic acid or salt thereof, which is the second complexing agent, serves to restrain an increase in pH value due to sulfite and maintains the weak acid range. As the organic carboxylic acid or salt thereof, citric acid, malic acid, lactic acid, and the like can be cited. The organic carboxylic acid or salt thereof preferably has a concentration in the range of 5 to 50 g/L. If the concentration is lower than 5 g/L, the stability of gold sulfite is poor. If the concentration is higher than 50 g/L, the specific gravity of liquid increases, so that the deposition rate decreases.
The EDTA or salt thereof, which is another second complexing agent, serves to stabilize the gold complex in sodium gold sulfite or ethylene diamine complex of sodium gold sulfite. As the salt of EDTA, EDTA-2Na (sodium), EDTA-2K (potassium) can be mainly cited. The EDTA or salt thereof preferably has a concentration in the range of 5 to 50 g/L. If the concentration is lower than 5 g/L, the stability of gold sulfite is poor. If the concentration is higher than 50 g/L, there is a tendency toward poor adhesion.
Furthermore, the displacement gold plating solution in accordance with the present invention preferably has a pH value in the weak acid range of 4.5 to 6.0. If the pH value is lower than 4.5, the stability of gold sulfite is poor. If the pH value is higher than 6.0, there is a tendency toward a decreased deposition rate. Also, the solution temperature is preferably in the range of 45 to 65xc2x0 C. If the solution temperature is lower than 45xc2x0 C., the rate of displacement reaction decreases. If the solution temperature is higher than 65xc2x0 C., there is a tendency toward self-decomposition of the plating solution itself, so that satisfactory displacement gold plating cannot be performed.
The displacement plating solution in accordance with the present invention enables displacement gold plating with good adhesion properties and high solderability in the case where the substrate has been electroless nickel-phosphorous plated or nickel-boron plated. Further, the displacement gold plating solution in accordance with the present invention can be used in the case where the substrate has been electrolytic nickel plated, in which it has so far been difficult to perform the displacement gold plating itself. Specifically, in the case where the surface of a subject to be plated has been electrolytic nickel plated, displacement gold plating with good adhesion properties and high solderability can be performed.